1. Field of the Invention
This invention relates to a process for improving the processibility of composite propellants. More particularly, this invention relates to a process for improving pot life of certain composite propellants by eliminating the catalytic effect of iron oxide on the curing agent/polymer reaction without adversely affecting the propellant burning rate accelerating characteristics of the iron oxide.
2. Description of the Prior Art
Solid composite propellants are composed largely of (1) organic polymer binder fuel; (2) inorganic oxidizers; (3) metallic powders such as aluminum; and (4) a burning rate catalyst, such as iron oxide, where higher mass discharge rates are required. Major polymer binders in common useage today are the polyurethane types which are based on hydroxyl-terminate polymers such as hydroxy-terminated polybutadiene, hydroxyl-terminated polyesters, and hydroxyl-terminated polyethers cured with di-or tri-functional organic isocyanate compounds. Other polymer binders of some significance are those based on carboxyl-terminated polymers such as carboxyl-terminated polybutadiene cured with di- or tri-functional aziridines and/or epoxide compounds. Ammonium perchlorate, ammonium nitrate, RDX, and HMX are commonly used as the oxidizing agents in composite propellants. Metallic powders such as aluminum are often added to these compositions to increase energy level and to obtain a greater propellant weight in a given volume loading. There are different commonly used methods of increasing propellant burning rate: (1) use finer oxidizer particles; (2) use finer metallic particles; (3) operate at higher motor chamber pressures; (4) use a burning rate catalyst.
Iron oxide is a commonly used burning rate catalyst in various composite propellant compositions. However, a significant loss in propellant pot life, resulting in poor processing properties, is regularly encountered when using this material. This loss in pot life is essentially due to the catalytic effect of the iron oxide on the cure rate of the polymer binder. The pot life problem is especially acute in those propellant compositions where the polymer binder is of the polyurethane type. The short pot life of these composite propellants containing iron oxide results in a rapid increase in propellant viscosity during processing which leads to the formation of trapped voids in the cast propellant and the subsequent rejection of large numbers of defective rocket motors or the production of rocket motors of inferior quality. It is therefore necessary in order to overcome this problem to eliminate or at least greatly reduce the catalytic effect of the iron oxide on the cure rate of the polymer binder without adversely affecting the burning rate modifying characteristics of the iron oxide.
It is accordingly a primary object of the present invention to improve the processing characteristics of composite propellants containing iron oxide. It is a further more specific object of the invention to improve the processing characteristics of composite propellants containing iron oxide by reducing or eliminating the catalytic effect of iron oxide on the cure rate of the propellant binder without affecting the burning rate modifying characteristics of the iron oxide.
Other objects and advantages of the present invention will become apparent to those skilled in the art in view of the following detailed description.